A new mouse model of metabolic syndrome and associated complications

Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome

Snyder-Robinson syndrome (SRS) is a rare X-linked recessive disorder caused by a mutation in the SMS gene, which encodes spermine synthase, and aberrant polyamine metabolism. SRS is characterized by intellectual disability, thin habitus, seizure, low muscle tone/hypotonia and osteoporosis. Progress towards understanding and treating SRS requires a model that recapitulates human gene variants and disease presentations. Here, we evaluated molecular and neurological presentations in the G56S mouse model, which carries a missense mutation in the Sms gene. The lack of SMS protein in the G56S mice resulted in increased spermidine/spermine ratio, failure to thrive, short stature and reduced bone density. They showed impaired learning capacity, increased anxiety, reduced mobility and heightened fear responses, accompanied by reduced total and regional brain volumes. Furthermore, impaired mitochondrial oxidative phosphorylation was evident in G56S cerebral cortex, G56S fibroblasts and Sms-null hippocampal cells, indicating that SMS may serve as a future therapeutic target. Collectively, our study establishes the suitability of the G56S mice as a preclinical model for SRS and provides a set of molecular and functional outcome measures that can be used to evaluate therapeutic interventions for SRS.

Endothelial leptin receptor is dispensable for leptin-induced sympatho-activation and hypertension in male mice

Leptin plays a crucial role in blood pressure (BP) regulation, notably in the context of obesity through central sympatho-mediated pressor effects. Leptin also relaxes arteries via endothelial (EC) leptin receptor (LepREC)-mediated increases in nitric oxide (NO) bioavailability. Herein, we investigated whether leptin-mediated increases in NO bioavailability represent a buffering mechanism against leptin-induced sympatho-activation. We tested the direct contribution of LepREC to BP regulation in physiological conditions and in response to chronic leptin infusion using mice deficient in LepREC. LepREC deficiency did not alter baseline metabolic profile nor leptin-induced reduction in adiposity and increases in energy expenditure. LepREC-/- mice demonstrated no increase in baseline BP and heart rate (HR) (MAP: LepREC+/+:94.7 ± 1.6, LepREC-/-:95.1 ± 1.8 mmHg; HR:LepREC+/+:492.4 ± 11.7, LepREC-/-:509.5 ± 13.4 bpm) nor in response to leptin (MAP, LepREC+/+:101.1 ± 1.7, LepREC-/-:101.7 ± 1.8 mmHg; HR, LepREC+/+:535.6 ± 11.1, LepREC-/-:539.3 ± 14.2 bpm). Moreover, baseline neurogenic control of BP and HR was preserved in LepREC-/- mice as well as leptin-mediated increases in sympathetic control of BP and HR and decreases in vagal tone. Remarkably, LepREC deficiency did not alter endothelium-dependent relaxation in resistance vessels, nor NO contribution to vasodilatation. Lastly, leptin induced similar increases in adrenergic contractility in mesenteric arteries from both LepREC+/+ and LepREC-/- mice. Collectively, these results demonstrate that the NO buffering effects of leptin are absent in resistance arteries and do not contribute to BP regulation. We provide further evidence that leptin-mediated hypertension involves increased vascular sympatho-activation and extend these findings by demonstrating for the first time that increased cardiac sympatho-activation and reduced vagal tone also contribute to leptin-mediated hypertension.

Serum and cecal metabolic profile of the insulin resistant and dyslipidemic p47phox knockout mice

Involvement of NOX-dependent oxidative stress in the pathophysiology of metabolic disorders as well as in the maintenance of metabolic homeostasis has been demonstrated previously. In the present study, the metabolic profile in p47^phox-/- and WT mice fed on a chow diet was evaluated to assess the role of metabolites in glucose intolerance and dyslipidemia under altered oxidative stress conditions. p47^phox-/- mice displayed glucose intolerance, dyslipidemia, hyperglycemia, insulin resistance (IR), hyperinsulinemia, and altered energy homeostasis without any significant change in gluconeogenesis. The expression of genes involved in lipid synthesis and uptake was enhanced in the liver, adipose tissue, and intestine tissues. Similarly, the expression of genes associated with lipid efflux in the liver and intestine was also enhanced. Enhanced gut permeability, inflammation, and shortening of the gut was evident in p47^phox-/- mice. Circulating levels of pyrimidines, phosphatidylglycerol lipids, and 3-methyl-2-oxindole were augmented, while level of purine was reduced in the serum. Moreover, the cecal metabolome was also altered, as was evident with the increase in indole-3-acetamide, N-acetyl galactosamine, glycocholate, and a decrease in hippurate, indoxyl sulfate, and indigestible sugars (raffinose and melezitose). Treatment of p47^phox-/- mice with pioglitazone, marginally improved glucose intolerance, and dyslipidemia, with an increase in PUFAs (linoleate, docosahexaenoic acid, and arachidonic acid). Overall, the results obtained in p47^phox-/- mice indicate an association of IR and dyslipidemia with altered serum and cecal metabolites (both host and bacterial-derived), implying a critical role of NOX-derived ROS in metabolic homeostasis.

Dusp26 phosphatase regulates mitochondrial respiration and oxidative stress and protects neuronal cell death

The dual specificity protein phosphatases (Dusps) control dephosphorylation of mitogen-activated protein kinases (MAPKs) as well as other substrates. Here, we report that Dusp26, which is highly expressed in neuroblastoma cells and primary neurons is targeted to the mitochondrial outer membrane via its NH2-terminal mitochondrial targeting sequence. Loss of Dusp26 has a significant impact on mitochondrial function that is associated with increased levels of reactive oxygen species (ROS), reduction in ATP generation, reduction in mitochondria motility and release of mitochondrial HtrA2 protease into the cytoplasm. The mitochondrial dysregulation in dusp26-deficient neuroblastoma cells leads to the inhibition of cell proliferation and cell death. In vivo, Dusp26 is highly expressed in neurons in different brain regions, including cortex and midbrain (MB). Ablation of Dusp26 in mouse model leads to dopaminergic (DA) neuronal cell loss in the substantia nigra par compacta (SNpc), inflammatory response in MB and striatum, and phenotypes that are normally associated with Neurodegenerative diseases. Consistent with the data from our mouse model, Dusp26 expressing cells are significantly reduced in the SNpc of Parkinson's Disease patients. The underlying mechanism of DA neuronal death is that loss of Dusp26 in neurons increases mitochondrial ROS and concurrent activation of MAPK/p38 signaling pathway and inflammatory response. Our results suggest that regulation of mitochondrial-associated protein phosphorylation is essential for the maintenance of mitochondrial homeostasis and dysregulation of this process may contribute to the initiation and development of neurodegenerative diseases.

Disruption of metabolic, sleep, and sensorimotor functional outcomes in a female transgenic mouse model of Alzheimer's disease

Alzheimer's Disease (AD) is the most prevalent form of dementia globally, and the number of individuals with AD diagnosis is expected to double by 2050. Numerous preclinical AD studies have shown that AD neuropathology accompanies alteration in learning and memory. However, less attention has been given to alterations in metabolism, sleep, and sensorimotor functional outcomes during AD pathogenesis. The objective of this study was to elucidate the extent to which metabolic activity, sleep-wake cycle, and sensorimotor function is impaired in APPSwDI/Nos2^-/- (CVN-AD) transgenic mice. Female mice were used in this study because AD is more prevalent in women compared to men. We hypothesized that the presence of AD neuropathology in CVN-AD mice would accompany alterations in metabolic activity, sleep, and sensorimotor function. Our results showed that CVN-AD mice had significantly decreased energy expenditure compared to wild-type (WT) mice. An examination of associated functional outcome parameters showed that sleep activity was elevated during the awake (dark) cycle and as well as an overall decrease in spontaneous locomotor activity. An additional functional parameter, the nociceptive response to thermal stimuli, was also impaired in CVN-AD mice. Collectively, our results demonstrate CVN-AD mice exhibit alterations in functional parameters that resemble human-AD clinical progression.

Glucose and lipid metabolism alterations in liver and adipose tissue pre-dispose p47phox knockout mice to systemic insulin resistance

Oxidative stress due to enhanced production or reduced scavenging of reactive oxygen species (ROS) has been associated with diet (dyslipidemia) induced obesity and insulin resistance (IR). The present study was undertaken to assess the role of p47^phox in IR using wild type (WT) and p47^phox-/- mice, fed with different diets (HFD, LFD or Chow). Augmented body weight, glucose intolerance and reduced insulin sensitivity were observed in p47^phox-/- mice fed with 45% HFD and 10% LFD. Further, body fat and circulating lipids were increased significantly with 5 weeks LFD feeding in p47^phox-/- mice, while parameters of energy homeostasis were reduced as compared with WT mice. LFD fed knockout (KO) mice showed an enhanced hepatic glycogenolysis, and reduced insulin signalling in liver and adipose tissue, while skeletal muscle tissue remained unaffected. A significant increase in hepatic lipids, adiposity, as well as expression of genes regulating lipid synthesis, breakdown and efflux were observed in LFD fed p47^phox-/- mice after 5 weeks. On the other hand, mice lacking p47^phox demonstrated altered glucose tolerance and tissue insulin sensitivity after 5 weeks chow feeding, while changes in body weight, respiratory exchange ratio (RER) and heat production are non-significant. Our data demonstrate that lack of p47^phox is sufficient to induce IR through altered glucose and lipid utilization by the liver and adipose tissue.

Cathepsin K knockout protects against cardiac dysfunction in diabetic mice

Diabetes is a major risk factor for cardiovascular disease and the lysosomal cysteine protease cathepsin K plays a critical role in cardiac pathophysiology. To expand upon our previous findings, we tested the hypothesis that, knockout of cathepsin K protects against diabetes-associated cardiac anomalies. Wild-type and cathepsin K knockout mice were rendered diabetic by streptozotocin (STZ) injections. Body weight, organ mass, fasting blood glucose, energy expenditure, cardiac geometry and function, cardiac histomorphology, glutathione levels and protein levels of cathepsin K and those associated with Ca²⁺ handling, calcineurin/NFAT signaling, insulin signaling, cardiac apoptosis and fibrosis were determined. STZ-induced diabetic mice exhibited distinct cardiac dysfunction, dampened intracellular calcium handling, alterations in cardiac morphology, and elevated cardiomyocyte apoptosis, which were mitigated in the cathepsin K knockout mice. Additionally, cathepsin K knockout mice attenuated cardiac oxidative stress and calcineurin/NFAT signaling in diabetic mice. In cultured H9c2 myoblasts, pharmacological inhibition of cathepsin K, or treatment with calcineurin inhibitor rescued cells from high-glucose triggered oxidative stress and apoptosis. Therefore, cathepsin K may represent a potential target in treating diabetes-associated cardiac dysfunction.

Altered glucose and lipid homeostasis in liver and adipose tissue pre-dispose inducible NOS knockout mice to insulin resistance

On the basis of diet induced obesity and KO mice models, nitric oxide is implied to play an important role in the initiation of dyslipidemia induced insulin resistance. However, outcomes using iNOS KO mice have so far remained inconclusive. The present study aimed to assess IR in iNOS KO mice after 5 weeks of LFD feeding by monitoring body composition, energy homeostasis, insulin sensitivity/signaling, nitrite content and gene expressions changes in the tissues. We found that body weight and fat content in KO mice were significantly higher while the respiratory exchange ratio (RER), volume of carbon dioxide (VCO₂), and heat production were lower as compared to WT mice. Furthermore, altered systemic glucose tolerance, tissue insulin signaling, hepatic gluconeogenesis, augmented hepatic lipids, adiposity, as well as gene expression regulating lipid synthesis, catabolism and efflux were evident in iNOS KO mice. Significant reduction in eNOS and nNOS gene expression, hepatic and adipose tissue nitrite content, circulatory nitrite was also observed. Oxygen consumption rate of mitochondrial respiration has remained unaltered in KO mice as measured using extracellular flux analyzer. Our findings establish a link between the NO status with systemic and tissue specific IR in iNOS KO mice at 5 weeks.

Ptp1b deletion in pro-opiomelanocortin neurons increases energy expenditure and impairs endothelial function via TNF-α dependent mechanisms

Protein tyrosine phosphatase 1b (Ptp1b) is a negative regulator of leptin and insulin-signalling pathways. Its targeted deletion in proopiomelanocortin (POMC) neurons protects mice from obesity and diabetes by increasing energy expenditure. Inflammation accompanies increased energy expenditure. Therefore, the present study aimed to determine whether POMC-Ptp1b deletion increases energy expenditure via an inflammatory process, which would impair endothelial function. We characterized the metabolic and cardiovascular phenotypes of Ptp1b+/+ and POMC-Ptp1b−/− mice. Clamp studies revealed that POMC-Ptp1b deletion reduced body fat and increased energy expenditure as evidenced by a decrease in feed efficiency and an increase in oxygen consumption and respiratory exchange ratio. POMC-Ptp1b deletion induced a 2.5-fold increase in plasma tumour necrosis factor α (TNF-α) levels and elevated body temperature. Vascular studies revealed an endothelial dysfunction in POMC-Ptp1b−/− mice. Nitric oxide synthase inhibition [N-nitro-L-arginine methyl ester (L-NAME)] reduced relaxation to a similar extent in Ptp1b+/+ and POMC-Ptp1b−/− mice. POMC-Ptp1b deletion decreased ROS-scavenging enzymes [superoxide dismutases (SODs)] whereas it increased ROS-generating enzymes [NADPH oxidases (NOXs)] and cyclooxygenase-2 (COX-1) expression, in aorta. ROS scavenging or NADPH oxidase inhibition only partially improved relaxation whereas COX-2 inhibition and thromboxane-A2 (TXA2) antagonism fully restored relaxation in POMC-Ptp1b−/− mice. Chronic treatment with the soluble TNF-α receptor etanercept decreased body temperature, restored endothelial function and reestablished aortic COX-2, NOXs and SOD expression to their baseline levels in POMC-Ptp1b−/− mice. However, etanercept promoted body weight gain and decreased energy expenditure in POMC-Ptp1b−/− mice. POMC-Ptp1b deletion increases plasma TNF-α levels, which contribute to body weight regulation via increased energy expenditure and impair endothelial function via COX-2 and ROS-dependent mechanisms.

Deletion of protein tyrosine phosphatase 1b in proopiomelanocortin neurons reduces neurogenic control of blood pressure and protects mice from leptin- and sympatho-mediated hypertension

Protein tyrosine phosphatase 1b (Ptp1b), which represses leptin signaling, is a promising therapeutic target for obesity. Genome wide deletion of Ptp1b, increases leptin sensitivity, protects mice from obesity and diabetes, but alters cardiovascular function by increasing blood pressure (BP). Leptin-control of metabolism is centrally mediated and involves proopiomelanocortin (POMC) neurons. Whether these neurons contribute to leptin-mediated increases in BP remain unclear. We hypothesized that increasing leptin signaling in POMC neurons with Ptp1b deletion will sensitize the cardiovascular system to leptin and enhance neurogenic control of BP. We analyzed the cardiovascular phenotype of Ptp1b+/+ and POMC-Ptp1b-/- mice, at baseline and after 7 days of leptin infusion or sympatho-activation with phenylephrine. POMCPtp1b deletion did not alter baseline cardiovascular hemodynamics (BP, heart rate) but reduced BP response to ganglionic blockade and plasma catecholamine levels that suggests a decreased neurogenic control of BP. In contrast, POMC-Ptp1b deletion increased vascular adrenergic reactivity and aortic α-adrenergic receptors expression. Chronic leptin treatment reduced vascular adrenergic reactivity and blunted diastolic and mean BP increases in POMC-Ptp1b-/- mice only. Similarly POMC-Ptp1b-/- mice exhibited a blunted increased in diastolic and mean BP accompanied by a gradual reduction in adrenergic reactivity in response to chronic vascular sympatho-activation with phenylephrine. Together these data rule out our hypothesis but suggest that deletion of Ptp1b in POMC neurons protects from leptin- and sympatho-mediated increases in BP. Vascular adrenergic desensitization appears as a protective mechanism against hypertension, and POMC-Ptp1b as a key therapeutic target for the treatment of metabolic and cardiovascular dysfunctions associated with obesity.

Aldolase-B knockout in mice phenocopies hereditary fructose intolerance in humans

The rise in fructose consumption, and its correlation with symptoms of metabolic syndrome (MBS), has highlighted the need for a better understanding of fructose metabolism. To that end, valid rodent models reflecting the same metabolism as in humans, both biochemically and physiologically, are critical. A key to understanding any type of metabolism comes from study of disease states that affect such metabolism. A serious defect of fructose metabolism is the autosomal recessive condition called hereditary fructose intolerance (HFI), caused by mutations in the human aldolase B gene (Aldob). Those afflicted with HFI experience liver and kidney dysfunction after fructose consumption, which can lead to death, particularly during infancy. With very low levels of fructose exposure, HFI patients develop non-alcoholic fatty acid liver disease and fibrosis, sharing liver pathologies also seen in MBS. A major step toward establishing that fructose metabolism in mice mimics that of humans is reported by investigating the consequences of targeting the mouse aldolase-B gene (Aldo2) for deletion in mice (Aldo2(-/-)). The Aldo2(-/-) homozygous mice show similar pathology following exposure to fructose as humans with HFI such as failure to thrive, liver dysfunction, and potential morbidity. Establishing that this mouse reflects the symptoms of HFI in humans is critical for comparison of rodent studies to the human condition, where this food source is increasing, and increasingly controversial. This animal should provide a valuable resource for answering remaining questions about fructose metabolism in HFI, as well as help investigate the biochemical mechanisms leading to liver pathologies seen in MBS from high fructose diets.

Adiponectin knockout accentuates high fat diet-induced obesity and cardiac dysfunction: role of autophagy

Adiponectin (APN), an adipose-derived adipokine, offers cardioprotective effects although the precise mechanism of action remains unclear. This study was designed to examine the role of APN in high fat diet-induced obesity and cardiac pathology. Adult C57BL/6 wild-type and APN knockout mice were fed a low or high fat diet for 22weeks. After 40day feeding, mice were treated with 2mg/kg rapamycin or vehicle every other day for 42days on respective fat diet. Cardiomyocyte contractile and Ca(2+) transient properties were evaluated. Myocardial function was evaluated using echocardiography. Dual energy X-ray absorptiometry was used to evaluate adiposity. Energy expenditure, metabolic rate and physical activity were monitored using a metabolic cage. Lipid deposition, serum triglyceride, glucose tolerance, markers of autophagy and fatty acid metabolism including LC3, p62, Beclin-1, AMPK, mTOR, fatty acid synthase (FAS) were evaluated. High fat diet intake induced obesity, systemic glucose intolerance, cardiac hypertrophy, dampened metabolic ability, cardiac and intracellular Ca(2+) derangements, the effects of which were accentuated by APN knockout. Furthermore, APN deficiency augmented high fat diet-induced upregulation in the autophagy adaptor p62 and the decline in AMPK without affecting high fat diet-induced decrease in LC3II and LC3II-to-LC3I ratio. Neither high fat diet nor APN deficiency altered Beclin-1. Interestingly, rapamycin negated high fat diet-induced/APN-deficiency-accentuated obesity, cardiac hypertrophy and contractile dysfunction as well as AMPK dephosphorylation, mTOR phosphorylation and p62 buildup. Our results collectively revealed that APN deficiency may aggravate high fat diet-induced obesity, metabolic derangement, cardiac hypertrophy and contractile dysfunction possibly through decreased myocardial autophagy.

ENU mutagenesis identifies mice with morbid obesity and severe hyperinsulinemia caused by a novel mutation in leptin

BACKGROUND

Obesity is a multifactorial disease that arises from complex interactions between genetic predisposition and environmental factors. Leptin is central to the regulation of energy metabolism and control of body weight in mammals.

METHODOLOGY/PRINCIPAL FINDINGS

To better recapitulate the complexity of human obesity syndrome, we applied N-ethyl-N-nitrosourea (ENU) mutagenesis in combination with a set of metabolic assays in screening mice for obesity. Mapping revealed linkage to the chromosome 6 within a region containing mouse Leptin gene. Sequencing on the candidate genes identified a novel T-to-A mutation in the third exon of Leptin gene, which translates to a V145E amino acid exchange in the leptin propeptide. Homozygous Leptin(145E/145E) mutant mice exhibited morbid obesity, accompanied by adipose hypertrophy, energy imbalance, and liver steatosis. This was further associated with severe insulin resistance, hyperinsulinemia, dyslipidemia, and hyperleptinemia, characteristics of human obesity syndrome. Hypothalamic leptin actions in inhibition of orexigenic peptides NPY and AgRP and induction of SOCS1 and SOCS3 were attenuated in Leptin(145E/145E) mice. Administration of exogenous wild-type leptin attenuated hyperphagia and body weight increase in Leptin(145E/145E) mice. However, mutant V145E leptin coimmunoprecipitated with leptin receptor, suggesting that the V145E mutation does not affect the binding of leptin to its receptor. Molecular modeling predicted that the mutated residue would form hydrogen bond with the adjacent residues, potentially affecting the structure and formation of an active complex with leptin receptor within that region.

CONCLUSIONS/SIGNIFICANCE

Thus, our evolutionary, structural, and in vivo metabolic information suggests the residue 145 as of special function significance. The mouse model harboring leptin V145E mutation will provide new information on the current understanding of leptin biology and novel mouse model for the study of human obesity syndrome.